1. Field of the Invention
The present invention relates to a document feeding apparatus which can repeatedly process documents by separating and automatically feeding the documents stacked on a document stacker one by one onto the platen glass plate of a reproducing machine and by returning, if necessary, the documents exposed on the platen glass plate again to the document stacker.
2. Description of the Prior Art
Generally speaking, some document feeding apparatus (ADF) of this kind can separate and feed the documents stacked on a document stacker one by one onto the platen glass plate of a reproducing machine and to discharge the documents exposed on the platen glass plate to a discharge dish outside of the machine. Another document feeding apparatus (RDF) can perform repeated feeding by returning the exposed documents to the document stacker. Since these document feeding apparatus are equipped at their paper feed aperture with a document separating mechanism, they cannot feed either documents which are so liable to be broken that cannot withstand passage through the separating mechanism or documents which are difficult to be separated by the separating mechanism. In order to feed such documents, therefore, those document feeding apparatus are frequently provided additionally in the prior art with a function (SDF) to automatically feed the documents one by one. For example, a paper feed aperture having no separating mechanism is additionally provided separately from the paper feed aperture of the ADF or RDF so that the documents may be set one by one in the paper feed aperture added. In another system, the paper feed apertures of the ADF or RDF and SDF are used in common so that the documents may be fed in the SDF mode each time they are set one by one by the user.
Despite of this fact, however, the former system has operation problems and a complicated construction because of two paper feed apertures, and the latter system still cannot be free from the damage of the documents because the documents will pass through the separating unit even in the SDF mode.
The separation at the paper feeding unit is a remarkably important function in a circulation type document feeding apparatus (RDF) in which copies are formed one by one in one circulation so that the documents are circulated as many times as the number of the formed copies. A so-called "bottom feed--top return" system has been adopted in the prior art as the paper feeding system excellent in that separating function This system repeats the operations to separate and feed the documents stacked on the document stacker one by one from the lowermost one to the processing unit and to return the exposed documents to the uppermost position of the documents being fed when they are returned to the document stacker. This bottom feed is divided into two techniques: one adsorbs and separates one document by making use of suction and the other frictionally separates the documents by using a paper feeding belt and a stop roller forced to contact the belt. The latter technique has a merit over the former in that the system can be produced at a lower cost and with a compact construction so that it can be adjusted and maintained more simply. This technique is represented by a document feeding apparatus, as shown in FIG. 15. In this apparatus, a document stacker 140 is inclined steeply downhill in the paper feeding direction and has its lower end portion provided with a paper feeding aperture 141. One document fed out from this aperture 141 is guided via a paper feed passage 142 onto the platen glass plate 143 of a reproducing machine. The document thus guided is conveyed to an exposure position by the forward rotation of a conveyor belt 144 which is disposed over the platen glass plate 143. The document thus conveyed to the exposure position is subjected to an exposure by reciprocating an exposing optical system 145. The exposed document is then conveyed back by reversing the conveyor belt 144 until it is discharged in the opposite direction to the feeding direction onto the document stacker 140 through a paper discharge passage 146 from a paper discharge aperture 147 which is formed above the paper feeding aperture 141. At last, the document thus discharged is returned to the uppermost one of the unprocessed documents on the document stacker 140. In this case, the unprocessed documents and the processed and restacked documents are separated by a partition 148, which is disposed just upstream of the paper feeding aperture 141. The partition 148 makes one turn in a counterclockwise direction, after the last one of the unprocessed documents has been restacked through the processing unit, to ride on the uppermost document. After this, the document having made one circulation is placed on that partition 148.
In the apparatus described above, however, the angle of the downhill of the document stacker 140 has to be steep so that the stacker 140 has a very large total height. Since the restacked documents are set in the paper feeding unit 141 after their natural drops, their leading edges are not well arranged. Moreover, the restacked documents may fail to drop completely because of bending or curling and may be caught in their dropping courses, thus seriously deteriorating the separating performance. Because of the steep angle of the downhill of the document stacker 140, the restacked documents may steal below the aforementioned partition 148. Since, moreover, the documents have to be fed back after they have been once exposed at the exposing position of the platen glass plate, another problem is that the document interchanging rate is lowered.